Prior art systems have been proposed for extracting the coherent component of the input signal, even though the desired signal is a discontinuous, low duty cycle and buried in noise of the same magnitude. Among the techniques previously employed include correlators, Fourier transform analyzers, phase lock loops, and counter processors. At present, phase lock loops and counter processors are the most prevalent. Phase locked loops work well on composite signals that have discontinuities, phase changes, phase jitter and noise an order of magnitude higher than the coherent signal. Phase locked loop systems are particularly adapted to such applications in that the gain of the tracking loop and its band width can be easily adjusted to fit the requirements of a particular situation. Once the locked loop system has locked onto the desired component, the rejection of unwanted noise components is very good.
Phase locked loop systems have certain limitations. Among them is that they can be disturbed easily while in operation by abrupt phase reversals and may completely lose lock of the coherent components when the duty cycle becomes low. The phase locked loop system has a very narrow captive range making it difficult to control the system to capture a coherent signal if the properties of the coherent signal include large and fast fluctuations in frequency compared to the capture range along with a low duty cycle.
Counter processors are particularly adapted to measure the coherent component in a composite signal where the properties of the coherent component are discontinuous, have phase changes and random noise an order of magnitude smaller than the coherent component of this composite signal. Counter processors are particularly well adapted to measuring the coherent component of the signal when the duty cycles are very low, i.e. a few tenths of a percent and when the coherent component includes a large and fast fluctuation in frequency. Order of magnitude changes in frequency at hundreds of kilohertz fluctuation rates are easily measured.
Counter processors have certain limitations. Among these limitations are that they are sensitive to the absolute random noise component amplitude. The noise amplitude generally must be maintained below a particular threshold level, and the coherent component must be maintained above a second threshold, i.e. approximately 2.5 times the noise threshold. If this criteria is not maintained, the counter processor may produce erroneous measurements. In order to prevent these erroneous measurements, the absolute amplitude of the composite signal must be properly adjusted to keep the noise below the noise threshold.